The present invention relates to pumps and more particularly to centrifugal blood pumps without the requirement of a rotating seal to protect the pump bearings from the pumped blood.
Delicate surgical procedures require that the site of surgery remain motionless during the surgical process. This made early heart surgery difficult to impossible as interruption of the heart's pumping action for the required length of time would be invariably fatal.
During the 1960s, prolonged and non-fatal stoppage of the heart became possible by use of newly developed "heart-lung" machines. These machines consisted of a mechanical blood pump combined with a blood oxygenator. They were capable of taking over the function of the natural heart and lungs for periods of up to several hours, enabling the development of techniques leading to today's extensive practice of open-heart surgery.
The first practical mechanical blood pumps used were peristaltic or "roller" pumps. The pumping action of a roller pump derives from the compression of a section of the flexible plastic tubing which carries the blood through the heart-lung machine. A moving roller presses the tubing against a semicircular platen, moving the blood forward in the tubing. The speed of the moving roller and the diameter of the tubing control the rate of blood flow.
Although the roller pump was and is simple and reliable, it has two characteristics which can endanger the patient undergoing surgery. First, if flow is inadvertently obstructed, the pressure produced by a roller pump may exceed the bursting strength of the tubing circuit. Second, if air is accidentally introduced into the tubing circuit, it will be pumped to the patient along with the blood. Either of these conditions may result in serious or fatal consequences to the patient.
In 1976, centrifugal blood pumps began to replace the roller pump as the "heart" of the heart-lung machine. The pumping action of a centrifugal pump derives from the rotation of an impeller within a pumping chamber. Pump pressure is controlled by the rotational speed of the impeller. At operational speeds, excessive pressure cannot be produced. Additionally, the centrifugal forces in the pump form a natural air trap and, with massive introduction of air, deprime and discontinue pumping altogether. These two safety features, and the lower blood damage caused by these pumps, is now widely recognized, and has led to their extensive use for open heart surgery.
In the early 1980s it was demonstrated that a mechanical blood pump could be used as a heart-assist pump for patients who could not be separated from the heart-lung machine following surgery. The readily available centrifugal blood pumps were quickly applied to this situation as well as to the more routine use during heart surgery.
The fragility of the blood presents several problems for the design of mechanical blood pumps. Excessive shear forces cause rupture of the red blood cells (hemolysis). High flow velocity rates are needed over local areas of friction (such as seals) to prevent points of high temperature which cause blood damage and the accumulation of clot deposits.
Application of rotational impeller motion by conventional shaft drives has not been practical due to the need for a sterile barrier between the pumped blood and the pump drive mechanism. For this reason, centrifugal blood pumps commonly utilize a magnetic coupling between the pump impeller (or impeller shaft) and the drive motor.
Previous centrifugal blood pumps have relied on conventional ball bearings to support the impeller shaft. A rotating seal was used to protect the bearings from contamination by the pumped blood. Some centrifugal blood pumps utilized magnets carried by the impeller, which was supported by bearings mounted on a stationary shaft. A shaft seal was also required to protect the bearings from contamination by the pumped blood.
Due to the corrosive nature of blood, shaft seals usually fail after a relatively short time, exposing the bearings to contamination. If the failure is not detected, bearings may overheat, causing damage to the blood. Blood damage can lead to hemolysis, clot formation and stroke. The short useful life of current centrifugal blood pumps mandates their frequent replacement and is the single most important problem yet to be solved with these devices.